Balance wheel for an electric watch



Feb. 9, 1960 P. E. BIEMILLER BALANCE WHEEL FOR AN ELECTRIC WATCH.

Filed Jan. 2. 1958 INVENTOR PHILIP E. BIEMILLER ATTORNEY United States Patent BALANCE WHEEL FOR AN ELECTRIC WATCH Philip E. 'Biemiller, Landisville, Pa., assignor to Hamilton Watch Company, Lancaster, Pa., a corporation of Pennsylvania Application January 2, 1958, Serial No. 706,764 Claims. (Cl. 310-27) This invention relates to balance wheels for electric watches particularly of the type having a permanent magnetic field and a moving coil carried by the balance wheel.

Heretofore, in the making of balance wheels for spring powered watches, the construction of the cross arm of the balance wheel was more or less unimportant. So long as the cross arm provided rigid support for the balance wheel rim from a central staff, there was little function for other than furnishing the necessary support for the rim. It is true that in some types of balance wheels the cross-arm was made of a metal such as Invar which had relatively no expansion or contraction under normal changes of temperature to which a wrist watch would be subjected. Certain other constructions such as an 8 formation to provide a certain amount of resiliency and shock resistance has also been built into the cross-arm of a balance wheel.

With the inception of the electric wrist watch particularly of the type where a permanent magnetic field is set up through which a coil carried by the balance wheel passes, it has been found that with the ordinary type of cross bar in a balance wheel, there are electric currents set up in the cross arm caused by the passage of the cross arm through the magnetic field. These currents, commonly known as eddy currents, themselves set up a magnetic field in the vicinity of the cross arm which is directly opposed to the magnetic field of the permanent magnets and which when passing through said permanent magnetic field, increases the need for energy to drive the balance wheel. The eddy currents which are induced in the cross arm tend to set up magnetic fields which, reacting with the field of the permanent magnets, opposes the motion of the cross arm through that field. It will thus be seen that the conception of the electric wrist watch having a permanent magnetic field there is need for a completely new design of balance wheel having a cross arm which will resist the formation of eddy currents.

Further the amplitude of the oscillation of the balance wheel brings the cross arm to a position of rest within the permanent magnetic field. Metals, commonly referred to as non-magnetic, have paramagnetic characteristics so that there will be a magnetic attraction between the material comprising the cross arm and the permanent magnet. Such an attraction which will come at the end points of the oscillation can have a decidedly detrimental effect on the isochronal characteristics of the watch resulting in poor timekeeping. By reducing the mass of the cross arm the magnitude of the attraction is diminished and the isochronal characteristics are improved.

It is the object of the present invention to form the cross arm of a balance wheel so as to minimize the generation of eddy currents within the cross arm.

It is a further object of the present invention to form the cross arm with a series of openings spaced so that the metal between the openings is of comparatively small cross-section thus increasing the resistance to the passage of electricity to effectively reduce the eddy currents within the cross arm.

It is a still further object of the present invention to form the cross arm of the balance wheel to minimize the magnetic field which results from the eddy currents induced in the cross arm by the passage of the cross arm through the permanent magnetic field of the watch.

It is a still further object of the present invention to reduce the mass of the metal of the cross arm so that the magnetic attraction between the magnets of the permanent magnetic field and the cross arm of the balance wheel, as it passes over these magnets, is minimized.

The invention is shown in the accompanying drawing in which:

Figure 1 is a top plan view of the balance wheel of an electric watch showing the position of the permanent magnets in dotted lines.

Figure 2 is a section on line 2-2.

Figure 3 is a plan view of a modification.

Figure 4 is a section on line 44.

Figure 5 is a plan view of a modification.

Figure 6 is a section on line 66.

Figure 7 is a plan view of a modification.

Figure 8 is a section on line 88.

Referring particularly to Figure 1, the balance wheel of an electric watch of the type having a permanent magnetic field is shown with a rim 10, a coil 12 and a cross arm 14. The cross arm 14 is joined to the rim of the balance wheel at its ends by soldering, welding or it may be made integral with the rim of the balance wheel. It is formed with a central opening 16 for the ordinary balance stafi (not shown). On each side of the balance stafi are a pair of symmetrical openings 18 formed so that the connecting webs 20 and 22 joining the ends of the cross bar with the central portion 24 are of small cross section.

A pair of permanent magnets 26 are shown in dotted lines and provide a permanent magnetic field through which the coil 12 passes. A shunt which overlies the coil and intensifies and localizes the magnetic field is not shown. The dotted line position of the cross arm shown at 30 shows the normal extent of oscillation of the balance wheel. It will be seen that at the end of the oscillating period the cross arm is practically over the permanent magnet and well within the magnetic field. This affects the motion of the balance wheel in two ways. First the movement of the cross arm through the magnetic field sets up eddy currents within the body of the cross arm and the eddy currents produce a magnetic field which will of necessity be in opposition to the field of the permanent magnets 26. The reaction of the two magnetic fields results in an absorption of energy from the balance wheel so that to get the necessary amplitude in the motion of the balance wheel it is necessary to supply more energy than would be needed if the magnetic field set up by the eddy currents can be reduced to a minimum.

By Ohms law, V/R=I, it will be seen that even if the voltage induced by the eddy currents is low, the resistance to the passage of these eddy currents through the cross bar can be small so as to produce a large current. By increasing the resistance, in this case by reducing a cross section of the cross arm to a minimum, the resistance can be increased and the resulting current decreased. This in turn will reduce the strength of the magnetic field set up by the eddy currents and result in a more efiicient time piece.

Secondly, it will be seen from inspection of the position of the cross arm at the end of its oscillation that it is within the permanent magnetic field. Due to the paramagnetic nature of many metals there is a certain amount of attraction between the magnets and the crossminimized. Corning at the end of the period of oscil-' lation, the attraction between the magnets and the cross arm can produce serious isochronal anomalies.

Referring to Figure 3, a modification is shown in which a. balance rim 40 is provided with the usual coil 42. and a cross arm 44. The cross arm 44 is a modification of .the cross arm 14- of the invention shown in Figure l,

and is shown having a pair of slender webs 46 joined by cross arms 48 and supporting a central portion 50 formed with a hole 52 for the balance stafi (not shown). The reduction of the cross arm to its smallest cross. sectional areas here provides the greatest amount of resistance for the eddy currents thus formed and reduces resulting magnetic fields to a minimmn.

In the modification shown in Figure 5, a balance wheel having a rim 60, a coil 62 and a cross arm 64 has the cross arm formed with a series of perforations or holes 66 which are staggered and in close proximity one to the other so that the webs 68 between the holes are of the smallest cross section permissible to maintain the desired strength. The central portion '70 of the cross arm is formed with the usual opening 72 for the balance staff (not shown).

Figure 7 shows a further, modification of a balance 'wheel having a trim St), a coil 82 and a cross arm 84. The cross arm is made substantially rectangular and is provided with a series of overlapping slots 86 which are so spaced as to make the webs 88 between the slots of minimum cross section. The cross arm is formed with a central opening 9i for the reception of a balance staif (not shown).

In the operation of a watch it is highly desirable that the balance Wheel have an oscillation approximately 235 in either direction from a point of rest. This results in the passage of the cross arm of the balance Wheel through the magnetic tied at least twice in each oscillation.

The passage of the arm through the magnetic field sets up eddy currents within the arm which, in turn, produce a magnetic field which will oppose the motion of the balance arm through the magnetic field of the watch. This opposition to the motion of the balance wheel requires the expenditure of energy and anything which can reduce the amount of energy required to operate the watch will result in a longer life for the battery.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The. present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope or the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States Letters Patent is:

l. A balance wheel for an electric watch having a permanent magnetic field, comprising a rim, a cross arm connected at each end to the rim and formed with a central hub portion to receive a supporting staff, and a coil carried by said rim, said cross arm being formed with a plurality of openings in those portions between the ends and the central hub, the webs between the plurality of openings being of minimum cross section to minimize the formation of eddy currents during the passage of the cross arm through the permanent magnetic field.

2. A balance wheel for an electric Watch having a permanent magnetic field, comprising a rim, a cross arm connected at each end to the rim and formed with a central hub portion to receive a supporting staff, and a coil carried by said rim, said cross arm being formed with a plurality of overlapping slots in those portions between the ends and the hub portion to both minimize the mass of the cross arm and to minimize the formation of eddy currents within the cross arm during its passage through the magnetic field.

3. A balance wheel for an electric watch having a permanent'magnetic field, comprising a rim, a coil .carricd by said rim, a crossarm connected at each endto said rim and formed with a central hub portion to receivea supporting stall, said crossarm being formed with a plurality of ribs between said central hub portion and said end.

4. A balance Wheel for an electric watch having a permanent magnetic field comprising a rim, a coil carried by said rim, a crossarm having a plurality of ribsbetween said central hub portion and said rim, said crossarm further having a plurality of bracing members be,- tween said ribs.

5. A balance wheel for an electric watch as described in claim 4 wherein said plurality of ribs are parallel.

References Cited in the file of this patent UNITED STATES PATENTS 

